18 Microscopic Histochemistry 



meet the standards previously mentioned, may give rise to 

 misleading results when applied to tissues. 



The applicabihty of procedures of analytic chemistry to 

 histochemical research can be tested by model experiments. 

 Such experiments attempt to carry out the identifying re- 

 actions under conditions more or less similar to those pre- 

 vailing in tissue sections. The first, rather primitive, model 

 experiment is credited to Altmann,^ who investigated the 

 differential staining reactions of various fatty substances im- 

 bibed by a piece of tissue paper. In more accurate experi- 

 ments the substance in question is dissolved or finely dis- 

 persed in agar, gelatin, or some similar substance. The sus- 

 pension can be smeared on shdes or allowed to gel, fixed 

 and embedded like any tissue block. Another clever tech- 

 nique has been devised by Coujard,- permitting the com- 

 parison of a large number of substances on a single slide. 

 The substances to be tested are dissolved in serum, dilute 

 gelatin, or egg-white or some other freely flowing protein 

 solution, and marks are made with a clean steel pen on a 

 carefully cleaned shde, using the solutions as ink. The use 

 of different symbols as marks for the different substances 

 (e.g., abbreviations of their names, chemical formulas) will 

 facilitate prompt and easy recognition of the marks. As soon 

 as the shdes are dry, they can be processed as if they were 

 smears. Coujard's method assures the chemical comparison 

 of many different test substances under strictly identical 

 conditions. 



Model experiments find a number of applications in histo- 

 chemistry, of which a few will be mentioned. 



1. Determining the chemical specificity of methods —Ex- 

 amples of this will be mentioned in the sections on nucleic 

 acids, lipids, and phenolic substances. Each simple histolog- 

 ical staining method with no known chemical background 



1. Altmann, R.: Die Elementarorganismen und ihre Beziehungen zu den 

 Zellen (Leipzig: Veit & Co., 1890). 



2. Coujard, R.: Bull, d'histol. appliq. a la physiol., 20:161, 1943. 



